Feeding-associated corticosterone peak in rats under various feeding cycles

Features of the anticipatory (prefeeding) corticosterone peak under restricted feeding were examined in rats by forcing them to various feeding schedules. Meal feeding appears specific in inducing the anticipatory corticosterone peak, since other regular manipulations such as daily cleaning of the rat cage and sampling of vaginal smear were without effect. Under meal intervals of 22,24, and 26 h, the phase relation of prefeeding hormone peak to meal was 30 min, 2.5 h, and 4.5 h, respectively. When the interval was extended to 30 h, a hormone peak appeared not immediately but several hours before meal, and when it was shortened to 18 h, hormone peak was not observed before meal but appeared when regular meal was omitted; the interval between a preceding meal and the hormone peak was similarly 18-20 h in both cases. On the other hand, a single meal feeding had a bidirectional immediate effect on plasma hormone levels, namely, a postprandial elevation of the basal hormone level and a postprandial reduction of the already elevated prefeeding hormone level under restricted feeding. Both effects were observed alternately when rats were fed twice a day for 2 wk. It is concluded that the 24-h pattern of plasma corticosterone level in rats under restricted feeding is determined primarily by the interaction of three different factors: a light- entrainable circadian rhythm, a separate oscillation associated with meal feeding, and a bidirectional effect of a single meal.